Water-based inkjet inks are well known but are characterized by slow drying and poor resistance qualities in comparison with UV cured inks. However, despite their numerous advantages, UV curable inkjet inks are well known to have problems due to the relatively high film thickness applied (10-12 microns), which causes problems to the printer as a result of an uneven ink build across the web and distortion of the substrate roll in the machine. It is also well known that a potential solution to this problem is the use of a water-based UV curable ink, which initially deposits a similar film thickness to 100% UV inks (10-12 microns), but when the inks have been dried prior to UV curing, the film thickness is reduced to only a few microns and therefore drastically limits this roll distortion, while maintaining many of the positive product resistance properties associated with UV curable inks.
However, both the UV curing process and the removal of water from the printed ink impart a high level of heat to the substrate which can cause shrinkage and distortion that limits the choice of available substrates to the printer.
One strategy to reduce the thermal load to the substrate is to use a UV LED curing lamp in place of a conventional medium pressure mercury arc lamp. However, the choice of suitable photoinitiators which have chromophores extending out to the region where the UV LED lamps emit light is known to be very limited, and this problem becomes further compounded as these compounds tend not to be water soluble in a water-based UV system to the level required to allow adequate curing to take place.
US 2012/0027964 discloses an ink-jet receptive article comprising a substrate having a coating thereon comprising a vinyl lactam polymer or copolymer. The coating is a solvent-based composition comprising a vinyl lactam polymer or copolymer, a solvent, a polymer resin diluent, and optionally a pigment.
WO 2010/029017 discloses the synthesis and use of thioxanthone photoinitiators which are both thioxanthone and acrylate functional. They are described for use in UV curing inkjet inks. EP 0081280 describes an aqueous photo polymerisable composition for the production of a screen printing stencil. The composition contains ethylenically unsaturated monomers which are dispersible, miscible, or soluble in water; tertiary nitrogen-containing compounds; a water-soluble colloid; and a thioxanthone photoinitiator. The exemplified compositions would not be suitable for use in water-based UV inkjet ink. EP 0079119 describes the synthesis and use of water-soluble thioxanthone photoinitiators. The photoinitiators may be used in compositions such as for use in screen stencil production. Green discusses the potential use of carboxymethoxy thioxanthone as a water-soluble photoinitiator (Arthur Green, Octel Chemicals Ltd., Polymers Painting Colour Journal, Oct. 12, 1994, volume 184, number 4358). However, despite these speculations, Irgacure 2959 (from BASF), which is a hydroxyketone photoinitiator, remains the material of choice for water-based UV systems. U.S. Pat. No. 3,966,573 discloses efficient photoinitiator systems based on benzophenone, an amine compound, and a halogenated hydrocarbon.
WO 2005/012448 describes water-based UV curable inkjet compositions based on a plurality of 1,2 and/or 1,3-diol groups along the polymer backbone, and having pendant photo cross-linkable groups attached thereto. WO 02/064689 describes water-based UV curable inkjet inks containing polyurethane diluted in water, a colorant, and a photoinitiator. WO 99/07796 discloses water-based flash UV curable inkjet ink compositions based on a water dispersible or water soluble acrylate polymerisable material with a polymerisable material that is capable of softening the substrate, and is resistant to hydrolysis. U.S. Pat. No. 5,623,001 discloses a UV curable inkjet composition comprising water, a water miscible UV curable polymerisable material, a photoinitiator, and a colorant. EP 2075293 discloses an ink-jet ink comprising water, a pigment dispersion, water-soluble polymerizing or cross-linking substance having an ethylenically unsaturated group, and a water-soluble photoinitiator. U.S. Pat. No. 8,071,665 discloses a single phase aqueous composition that is suitable for inkjet printing. The composition comprises a mixture of curable materials including at least two curable oligomers, and one or more co-solvents for the curable materials. The curable materials are fully in solution. EP 1469049 teaches that the open time of a UV curable water-based inkjet ink may be improved through the use of a water-soluble UV monomer as a humectant.
GB 2256874 discloses a photocurable composition comprising a water dilutable multifunctional polyurethane acrylate oligomer, a multifunctional acrylate monomer, water, and a vinyl ether monomer. The compositions are useful as screen printing inks. US 2002/0065335 describes an aqueous photocurable composition comprising at least water, a polymerisable material, and a water-soluble photopolymerisation initiator. The application covers only cleavage type photoinitiators. EP 2703459 and EP 2703 458 disclose a photocurable ink composition containing colorant, water, photo-initiator, UV curable polyurethane dispersions, hydrophobic radiation-curable monomers and water-soluble or water-miscible radiation curable monomers. A printing method is also disclosed. US 2012/0225968 teaches the use of a urethane acrylate oligomer for use in solvent- and water-based inks. The ink compositions contain, in addition to the urethane acrylate, a compound having radical polymerizable groups and a photoradical polymerization initiator. US 2002/0121631 teaches UV curable overprint varnishes that can be applied in-line over conventional lithographic inks. The use of benzophenone and an amine synergist is shown to be an effective combination of photoinitiators. U.S. Pat. No. 5,395,863 discloses UV cured screen inks. The UV curable compositions include a free radically polymerizable monomer or prepolymer, and a different monomer containing an N-vinyl group. The compositions include a photoinitiator when they are to be cured by exposure to UV light, and can also include a photo accelerator.
Up to now, most attempts to develop improved water-based UV curable ink and coating compositions use either Irgacure 2959 as a photoinitiator (solubility in water 1.0%), look to develop more water soluble photoinitiators, particularly cleavage type, or use standard photoinitiators in an emulsion water-based UV system.
What is clear is that currently, there is no suitable approach to formulating a water-based UV inkjet ink that can be cured using a UV LED lamp.